When treating a subterranean formation which is sensitive to water, it is often necessary to minimize the amount of water in the well treatment fluid. In such instances, it is typically preferred to mix a gas with the treatment fluid. This allows for a reduction in the amount of water without loss of treatment fluid volume. In some instances, the gas is mixed with the treatment fluid in order to assist in the recovery of the fluid after treatment of the formation is concluded. Suitable gases include nitrogen and carbon dioxide. In some cases, a mixture of such gases may be used. A mixture of two of such gases is referred to as a binary composition.
Typically, when the amount of gas in the treatment fluid exceeds 30% by volume, a foaming agent is further added to the fluid in order to create a foamed fluid. The addition of a foam typically increases the viscosity of the treatment fluid. Where the amount of gas and foaming agent in the treatment fluid is greater than 63 volume percent, the fluid is termed as being “energized”. In addition to increasing viscosity, the foaming agent further contributes to the stability of the resulting fluid. Typical foaming agents include surfactants based on betaines, alpha olefin sulfonates, sulfate ethers, ethoxylated sulfate ethers and ethoxylates. Alpha olefin sulfonates are often preferred since they exhibit greater tolerance to oil contamination, such as that which originates from hydrocarbon based polymer slurries.
Often, well treatment fluids, such as those used as fracturing fluids, are either non-viscosified aqueous-based solutions (usually introduced into the formation at a high pumping rate) or are viscosified aqueous-based fluids. Hydraulic fracturing is the process of enhancing oil and/or gas production from producing wells or enhancing the injection of water or other fluids into injection wells. Fracturing fluids are usually injected into the wellbore and pass down tubulars within the wellbore into the formation. Once natural reservoir pressures are exceeded, the confining stresses in the formation cause the formation to fail, thereby inducing a fracture. A proppant is then deposited in the fracture, where it remains after the treatment is completed. The proppant serves to hold the fracture open, thereby enhancing the ability of fluids to migrate from the formation to the wellbore through the fracture.
Most viscosified fluids used in fracturing fluids are composed of water-soluble polymers which have been hydrated in water and chemically modified with crosslinking agents in order to increase fluid viscosity. Typical of such water-soluble polymers are those based on guar gum and include guar derivatives as well as cellulosic derivatives, xanthan and carrageenan. Commonly used crosslinking agents are those capable of providing borate ions as well as those agents which contain a metal ion such as aluminum, zirconium and titanium. Such viscosified fluids form three-dimensional gels. The viscosity of such fluids is sufficient to adequately carry and place proppant into the formation.
Further, viscosified fluids have been used in blocking gels. Such gels are used to isolate and protect the production zone of the well from damage. For instance, production from wellbore operations is typically temporarily ceased in order to perform such auxiliary procedures as workover operations. Such operations frequently use heavy brines and other fluids to maintain pressure control within the reservoir. Such fluids can leak-off into the production zone, causing damage which interferes with the efficient operation of the well. Temporary blocking gels are thus used to isolate and protect the production zone. Blocking gels formed by viscous fluids produce a relatively impermeable barrier across the production formation. The barrier cordons off the production zone from the area undergoing the workover operations until production is resumed after removal of the blocking gel.
In those instances where it is desired to use a foaming agent in order to treat a water sensitive formation, it has been found that many crosslinking agents are less effective in the presence of certain foaming agents, such as alpha olefin sulfonates. The ultimate effect is a substantial loss of foam viscosity. While loss in viscosity may be overcome by the addition of greater amounts of crosslinking agent to the polymer solution, the amount of additional crosslinking agent which must be added is often staggering. For instance, in some cases, the crosslinker concentration must be increased by 300%. This causes a substantial increase in the treatment cost.
More cost effective methodologies have therefore been sought for foamed well treatment fluids. Such alternative methodologies must be capable of providing foamed well treatment fluids having the requisite viscosity for the desired operation.